Study Of By CNT-supported/promoted Pt Catalyst For Hydro-dearomatization Of Toluene

The saturation of aromatic compounds in distillate fractions and in particular in diesel fuel has received considerable attention in recent years.A high aromatic content is associated with poor fuel quality,giving a low cetane number in diesel fuel and a high smoke point in jet fuel.New legislation has been introduced to limit aromatics in diesel fuel and this has led to new catalyst and process developments.Industrial hydro-dearomatization catalytic processes have been largely carried out on noble metals/Al₂O₃ hydrogenation catalysts.Notable features of alumina supports include their ability to provide high dispersion of the active metal components.However,the results have also shown that numerous chemical interactions exist between the amorphous alumina and transition metal oxides in the precursor state.Multi-walled carbon-nanotubes(MWCNTs,simplified as CNTs)have been drawing increasing attention recently.This new form of nanostructured carbon has a much higher degree of structural perfection,and possesses a series of unique features, such as,highly graphitized tube-wall,nanosized channel and sp²-C-constructed surface.They also display high thermal and electrical conductivity,and excellent performance for adsorption and spillover of hydrogen,which make this kind of nanostructured carbon materials full of promise as a novel catalyst support or promoter.In this dissertation,highly active CNT-supported Pt catalysts were prepared by the conventional incipient wetness method.Their catalytic performance for hydro-dearomatization(HDA)of toluene was evaluated,and compared with the reference systems based onγ-Al₂O₃ and activated carbon(AC).Using a number of physico-chemical methods(TEM/SEM/EDX,XRD,XPS,H₂-TPR,H₂-TPD,etc), these catalyst systems were characterized.The results shed some light on understanding the nature of promoter action by the CNTs and the prospect of developing highly active catalysts.The progresses obtained in the present work were briefly described as follows: 1.Preparation and performance of CNT-supported/promoted Pt catalyst for HDA of toluene1)The optimization test of Pt-loading amount showed that over the eight catalyst samples with different Pt-loading amounts under reaction condition of 0.4 MPa, 373 K,C₇H₈/H₂ = 6/94(mol/mol),GHSV = 1.2×10⁵ ml_STP·h^-1·g_-cat.^-1,the observed sequence of reactivity of toluene HDA is as follows:1.2%Pt/CNTs≈1.1%Pt/CNTs≈1.0%Pt/CNTs>0.9%Pt/CNTs>0.8%Pt/CNTs>0.7%Pt/CNTs> 0.5%Pt/CNTs>0.3%Pt/CNTs,with～1.0%(mass percentage)of Pt-loading amount optimal.2)It was experimentally demonstrated that the support could significantly affect the catalyst activity for the HDA reaction.Over 1.0%Pt/CNTs catalyst under the reaction conditions of 0.4 MPa,373 K,C₇H₈/H₂=6/94(mol/mol),GHSV= 1.2×10⁵ml_STp·h^-1·g_-cat.^-1,the observed conversion of toluene HDA reached 100%, the corresponding specific reaction rate was 1.74 mmol_C7H8·s^-1·mmol_Pt^-1.This value was 1.79 and 1.27 times as high as that(0.97 and 1.37 mmol_C7H8·S^-1·mmol_Pt^-1)of the counterparts supported byγ-Al₂O₃ and AC, respectively,and was 1.48 and 2.38 times as high as that(1.17 and 0.73 mmol_C7H8·s^-1·mmol_Pt^-1)of the catalysts supported byγ-Al₂O₃ and AC with the respective optimal Pt-loading amount(1.4%Pt/γ-Al₂O₃ and 2.4%Pt/AC), respectively.2.Characterization of CNT-supported/promoted Pt catalyst for HDA of toluene1)XRD measurement of the functioning catalyst of 1.0%Pt/CNTs catalyst showed that,except the features present at 2θ=26.1°,43.1°and 53.5°,which were due to the diffractions of(002),(100)and(004)planes,respectively,of graphitized tube-wall of the CNTs,the peaks due to Pt were detected at 20 = 39.7°,46.3°,and 67.3°,which were due to the diffractions of(111),(200),and(220)planes of metallic Pt.Those peaks,especially the one at 2θ= 39.7°,were considerably weak,and obviously weaker than that of the counterpart based onγ-Al₂O₃, implying that the supported Pt component was highly dispersed at the surface of CNTs,and existed mostly in states of smaller micro-crystalline,even non-crystalline.2)The H₂-TPR profiles taken on the three precursors of catalysts supported by the CNTs,AC andγ-Al₂O₃,respectively,with the sane Pt-loading amount showed that the position(～498 K)of H₂-reduction peak of the 1.0%Pt/CNTs catalyst was somewhat lower than that(506 K or 531 K)of the system based on AC orγ-Al₂O₃,while its specific H₂-consumed amount(i.e.,the amount of hydrogen consumed due to reduction of unit mass of Pt)was 1.28 and 1.61 times as high as that of the systems based on AC andγ-Al₂O₃ respectively,indicating that the reducibility of the CNT-supported catalyst was much higher than that of the reference system supported by AC orγ-Al₂O₃.3)XPS measurements revealed that using the CNTs in place of AC orγ-Al₂O₃ as support of the catalyst led to a slight increase in concentration of catalytically active Pt-species(Pt⁰)at the surface of the functioning catalyst.On the functioning 1.0%Pt/CNTs catalyst,the observed molar percentage of Pt^o-species in the total surface Pt-amount reached 97.3%.This value is 1.12 times as high as that(86.8%)of the counterpart supported by AC.Compared to the AC support, the CNTs with highly graphitized surface seem to be more effective in mitigating chemical interactions between the supported Pt component and the support to inhibit formation of some of Pt species,which are very stable and difficult to be reduced.4)H₂-TPD tests of the pre-reduced catalysts revealed that the CNT-supported Pt-catalyst could reversibly adsorb a greater amount of hydrogen under atmospheric pressure at temperatures from room temperature to 773 K,compared to the counterpart supported by AC orγ-Al₂O₃.The ratio of the relative area-intensities of the observed H₂-TPD profiles in the range of 323～773 K was as follows:S_1.0%Pt/CNTs/S_1.0%Pt/AC/S_{1.0%Pt/γ-Al2O3}=100/30/21;this sequence was in line with the observed reaction activity of toluene HDA over these catalysts. 3.Nature of the promoting action by the CNT-supportBased upon the aforementioned results about the preparation study of catalyst and the assay of HDA activity,as well as the characterization of the functioning catalysts,it has been demonstrated that the CNT-material could serve not only as cartier,but also as an excellent promoter,for the Pt-based catalyst for HDA of toluene. It was experimentally shown that using the CNTs in place of AC orγ-Al₂O₃ as support of the catalyst caused little change in the apparent activation energy for toluene HDA reaction,likely implying that the major reaction pathway of the toluene hydrogenation-conversion was unchanged,however,it led to a slight increase in concentration of catalytically active Pt-species(Pt⁰)at the surface of the functioning catalyst.In addition,the Pt/CNTs catalyst could reversibly adsorb a greater amount of hydrogen under atmospheric pressure at temperatures from room temperature to 573 K.This unique feature would help to generate microenvironments with higher stationary state concentration of active hydrogen-adspecies at the surface of the functioning catalyst.Both factors mentioned above were favorable to increasing the rate of toluene HDA reaction.